(1) Field of the Invention
The present invention relates to a device and a method of regulating a power plant including at least one turbine engine, and to an aircraft such as a rotorcraft.
(2) Description of Related Art
A rotorcraft conventionally includes a main lift or indeed propulsion rotor in the context of a helicopter. The main rotor is driven by a system referred to below as a “power plant”, for convenience.
A power plant includes at least one engine provided with a gas generator and with a free turbine for driving the main rotor in rotation. Such a gas generator is provided with a compressor connected to a high-pressure turbine, the high-pressure turbine being arranged upstream from the free turbine.
The driving power is then taken from a low-pressure stage of each free turbine, which stage is mechanically independent of the compressor assembly and of the high-pressure stage of the turbine engine. Each free turbine of a turbine engine operates at a speed of rotation lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reduction gearbox is needed in the connection with the main rotor since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is referred to as the main power transmission gearbox.
In order to control the operation of an engine, the aircraft includes an engine computer, known in particular under the acronym FADEC for “full authority digital engine control”.
In addition, the aircraft also includes air extraction means that are capable of extracting air from the gas generator of an engine. Such air extraction means may include a valve for adjusting the rate of extraction.
Furthermore, the aircraft includes mechanical power take-off means for taking off power mechanically from the gas generator.
The mechanical power take-off means may comprise electrical equipment that is connected to the gas generator of at least one engine. Such electrical equipment thus performs the function of an electricity generator by taking off power mechanically from the gas generator.
The electrical equipment may also perform a starter function by driving the gas generator during a starting stage.
The electrical equipment is thus sometimes referred to as a “starter-generator”.
In another aspect, thermal limitations on the engine and torque limitations on the main power transmission gearbox serve to define three normal utilization ratings for the engine.
Among known ratings, mention may be made of the following:
the take-off rating which associates a maximum take-off power PMD with a duration of utilization of about 5 minutes (min) to 10 min;
the maximum continuous rating associating a maximum continuous power PMC with an unlimited utilization duration; and
the transient rating associating a maximum transient power PMT with a limited utilization duration.
There also exist super-contingency ratings for aircraft having at least two engines, these ratings being for use when one of the engines fails:
a first contingency rating associates a super-contingency power with a duration of about thirty consecutive seconds known as 30 sec OEI (for one engine inoperative), this first contingency rating being usable on about three occasions during a flight;
a second contingency rating associating a maximum contingency power with a utilization duration of about two minutes, known as 2 min OEI; and
a third contingency rating associating an intermediate contingency power with a utilization duration extending to the end of a flight after one engine has failed, for example, known as continuous OEI.
Each power rating of an engine is thus monitored in use by means of monitoring parameters.
Since the engine is provided with a turbine assembly comprising a high-pressure turbine and a low-pressure turbine, a monitoring parameter may be a temperature of the gas flowing through the assembly.
In particular, since a high-pressure turbine is arranged upstream from a free turbine, a first monitoring parameter may be the temperature of the gas at the inlet to the high-pressure turbine, known as TET by the person skilled in the art.
The blades of the high-pressure turbine of the engine are subjected to centrifugal force and to the temperature TET. Above a certain level, the material constituting the blades is subjected to creep, thereby causing expansion that lengthens the blades. Thus, the blades run the risk of touching the casing of the high-pressure turbine and of thus being degraded. The temperature TET is thus associated directly with degradation of the engine.
Nevertheless, since the temperature TET is difficult to measure because of its relatively non-uniform nature, the first monitoring parameter may be the temperature of the gas at the entry to the free turbine, known to the person skilled in the art as T45. The temperature T45 is a good indicator of the temperature TET, and consequently it is representative of the degradation of the engine.
A first monitoring parameter is thus the temperature of an assembly having at least one turbine, this temperature possibly being the temperature TET of the gas at the inlet to the high-pressure turbine or the temperature T45 of the gas at the inlet to the free turbine.
In addition, a second monitoring parameter relates to the speed of rotation of the gas generator of the engine, known as Ng by the person skilled in the art.
A third monitoring parameter may be the torque Tq exerted by an outlet shaft of the engine driving a main power transmission gearbox.
The torque is monitored in particular to guarantee the physical integrity of the transmission shaft or of the main power transmission gearbox.
Under such circumstances, each power rating of an engine is associated with a first limitation of the first monitoring parameter and with a second limitation of the second monitoring parameter so as to preserve the health of the engine. Furthermore, each power rating is associated with a third limitation of the third monitoring parameter so as to comply the torque that is acceptable for the mechanical systems driven by the free turbine of the engine.
When a particular power rating of an engine is engaged, the power developed by the engine is thus limited by at least one of said limitations. The acceleration capacity of an engine is also limited by the fuel flow rate that can be supplied to the engine.
In addition, on an aircraft including at least two engines, when the third contingency rating is activated following the failure of one engine, it is still possible to extract air and to take off power mechanically from each engine in operation.
However, when the first contingency rating or the second contingency rating is activated, off-loading is necessary. Air extraction is conventionally stopped on the engines in operation.
Documents FR 2 914 697 and US 2010/0058731 relate to a turbine engine having a gas generator and a free turbine that is rotated by the stream of gas generated by the gas generator. The turbine engine further includes an auxiliary motor coupled to a shaft of the gas generator in order to supply an additional quantity of rotary kinetic energy to the shaft.
Furthermore, those documents envisage taking off a quantity of rotary kinetic energy from the shaft of the gas generator during a deceleration stage of the engine so as to assist the deceleration.
The technological background includes the following documents: EP 2 006 202; US 2004/119293; U.S. Pat. No. 4,736,331; U.S. Pat. No. 5,873,546; FR 2 902 407; and FR 2 968 716.